According to the US Geological Survey estimates, the Arctic region, mostly offshore, holds as much as 25% of the world's untapped reserve of hydrocarbons. Therefore, petroleum producers have shown significant interests in exploring oil and gas reserves in Arctic regions, particularly with the depletion of conventional hydrocarbon reservoirs. The areas of interest for oil and gas production mainly include Barents Sea, the Russian arctic, onshore Russia, Chukchi Sea, Beaufort Sea, the Canadian arctic islands, northern Canada, and the east coast of Greenland.
Arctic areas are typically overlain by substantial permafrost layers on the order of 150 to 500 meters thick, which can be continuous from the surfaces, or discontinuous with intermittent unfrozen zones. To mobilize cold hydrocarbon deposits, heat is added to the reservoir until the hydrocarbons are fluid enough to be pumped to the surface. Commonly used in situ extraction thermal recovery techniques include a number of steam-based heating methods, such as steam flooding or steam drive (SD), cyclic steam stimulation (CSS) or “huff-and-puff”, and Steam Assisted Gravity Drainage (SAGD), as well as various derivatives of these techniques.
SAGD is the most extensively used enhanced oil recovery technique for in situ recovery of bitumen resources in the McMurray Formation in the northern Alberta oil sands and other reservoirs containing viscous hydrocarbons. In a typical SAGD pattern, two horizontal wells are vertically spaced by 4 to 10 meters (m). The production well is located near the bottom of the pay and the steam injection well is located directly above and parallel to the production well. In SAGD, steam is injected continuously into the injection well, where it rises in the reservoir and forms a steam chamber. The heat from the steam reduces the oil's viscosity, thus enabling it to flow down to the production well and transported to the surface via pumps or lift gas.
As its name implies, generation of high quality, high temperature and high pressure steam is a prerequisite for the SAGD process. Specifications for the steam used for SAGD are 100% quality, 7,000-11,000 kPa pressure and 238° C.-296° C. temperature. Steam capacity (flowrate) is determined by the steam-to-oil (SOR) ratio which normally ranges around 2˜4. Considering oil production volume (10,000˜100,000 BPD depending on the well size), water requirement for steam generation is immense and the cost to create high quality steam is also highly significant.
As the steam is being transported to the payzone, heat is lost, causing the permafrost around the well to melt and resulting in settling of the soils due to the thaw. Thaw settlement becomes significant when steam carrying wells increase the temperature of surrounding soil and create a permafrost thaw bulb (see e.g., FIG. 1), thus reducing load carrying capacity of the soil and damaging surface structures. Permafrost thawing may have even more significant effects if areas of low thaw settlement (sands and gravels) are adjacent to areas sensitive to settlement (silts and clays), creating differential settlement. This differential settlement overstresses the well tubing and induces pipe bending strain and can result in loss of a well. As most thermal development takes place on pads, over time the thaw bulbs propagated from a single well can coalesce and form a thaw slot. Thaw slots introduce additional stresses, which can accelerate damage to the wells.
Methods to limit heat loss from wellbores have included the use of gelled diesel or insulated packer fluids in an annulus and/or using cement with higher thermal insulation properties. However, these methods have been less than satisfactory.
Another method of limiting heat loss uses vacuum insulated tubing (VIT), a typical sample of a VIT with partial cutaway is shown in FIG. 2. Vacuum insulated tubing has been widely used to limit heat loss from a wellbore, but its effectiveness has been limited, primarily due to the couplings. Couplings are a hot spot in that they do not have the same insulation properties as the main body of the joint. Accelerated heat loss can also happen when single or multiple joints of VIT lose their vacuum properties and lose heat close to the rate of a regular joint of tubing.
Thus, what are needed in the art are better methods of insulating wellbores in the Artic regions and other areas where temperature control is a concern.